The present invention relates to a highly active Ziegler-Natta catalyst for polymerizing HDPE and LLDPE and a process for polymerizing HDPE (high density polyethylene) and LLDPE (linear low density polyethylene) having various physical properties at low temperature and pressure by activating the catalyst component with an organoaluminum compound.
The preparation of ethylene polymers and copolymers having high density and large melt indices is described in European patent publication Nos. 0012147 and 0012148. This prior art method requires high polymerization temperature and pressure to prepare such polymers, as well as, a high ratio of hydrogen/ethylene in the reactor in order to maintain the melt indices above 1.0 g/10 minutes. However, there is a drawback in that the activity of catalysts is decreased when polymerization temperature is above 90.degree. C. and the ratio of hydrogen to ethylene becomes larger.
U.S. Pat. No. 4,124,532 discloses a process for polymerizing ethylene and propylene by using a highly active catalyst. The catalyst consists of complexes containing magnesium and titanium. The complexes are obtained by reacting metal chlorides such as MgCl.sub.2 with electron donor compounds having the formula M'Y, in which M' is a metal such as Ti, and Y is a halogen or an organic radical. The resulting complexes are isolated into solids by crystallization involving evaporating the solvent in which the reaction is carried out, or by precipitation. Such isolated complexes are used to prepare polymerization catalysts by means of reacting the complexes with an organoaluminum compound. However, when the liquid phase complexes are isolated by crystallization or precipitation as disclosed in the above patent, the particles will be very heterogeneous, therefore, there is a disadvantage in that the particles of ethylene polymer and copolymer polymerized by the catalyst complexes will also be heterogeneous. Furthermore, the polymerization rate of such catalysts tends to decrease with time. Thus, there are disadvantages in that the mean polymerization rate or activity, (Kg polymers/g-titanium.times.hr) does not reach the desirable level and the optimum concentration of organic aluminum is high, in the case of long term polymerization. Further, the undesirable hydrogenation of ethylene tends to occur in the processes of polymerization forming ethane which accumulates in the reactor to consequently reduce the concentration of ethylene further decreasing the activity. U.S. Pat. No. 2,198,077 discloses that the liquid phase complex obtained in the manner as described in U.S. Pat. No. 4,124,532 was supported on the inert carrier such as silica to prepare the supported catalyst complex and the resulting catalyst complex was activated with an organoaluminum compound in the processes of polymerization and used in the gas phase polymerization for a fluidized bed-reactor. The particle shape of ethylene polymer and copolymer produced with such prepared catalyst are excellent, but the processes for preparing the catalyst are complicated, and their activity is too low to be polymerized at low pressure for the slurry reaction and also they require a large amount of an organoaluminum compound. Therefore, they are not suitable.